The present invention refers to a method for determining NADH and/or NADPH-concentrations, for instance in NADH- or NADPH transforming systems, the method comprising the step of bringing the sample subject to determination in contact with the bioluminescent reagent based on bacteria luciferase, NAD(P)H-FMN oxidoreductase, FMN and aliphatic aldehyde, whereby a reaction takes place, where NAD(P)H is oxidized and FMN is reduced, the reaction being catalyzed by the oxidoreductase, whereafter the FMNH.sub.2 formed and the aldehyde are bound to the luciferase and light is emitted, the intensity of this light being measured and constituting a measure of the NADH- or NADPH-concentration.